A driving system for a work machine, such as a hydraulic excavator, may include a hybrid type drive system that has an electric generator, which is adapted to be driven by an engine, and an electric power storage device for storing electric power generated by the generator. An electric motor or a motor generator is operated by power supplied from either one of or both the generator and the electric power storage device and drives a pump or a pump motor.
For example, a boom control circuit for controlling a boom cylinder is adapted to drive a pump motor by operating a motor generator by means of electric power supplied from the generator or the electric power storage device. A stick control circuit for controlling a stick cylinder is adapted to drive a stick pump, i.e. a pump for a stick, by operating a stick motor, i.e. a motor for a stick, by means of electric power supplied from the generator or the electric power storage device. A bucket control circuit for controlling a bucket cylinder is adapted to drive a bucket pump by operating a bucket motor by means of electric power supplied from the generator or the electric power storage device. The boom control circuit, the stick control circuit, and the bucket control circuit are connected to one another by a plurality of supporting circuits that serve to feed hydraulic fluid to one another.
A boom cylinder driving circuit is a closed circuit including a bi-directional type pump motor and a motor generator. The bi-directional type pump motor is adapted to function as a pump for feeding hydraulic fluid and also function as a hydraulic motor driven by hydraulic fluid fed thereto. The motor generator is adapted to be driven by electric power supplied from the generator or the electric power storage device so as to function as an electric motor for driving the pump motor and also adapted to be driven by the pump motor so as to function as a generator for generating electric power (e.g. See Japanese Laid-open Patent Publication No. 2004-190845 (page 1, page 7, and FIG. 1)).
Whereas the boom control circuit requires a high flow rate the bucket control circuit requires a high pressure. Therefore, it is difficult to control the plurality of supporting circuits so that they satisfy these requirements.
Furthermore, the aforementioned combination of the pump motor and the motor generator is limited to a closed circuit and cannot be applied to an open circuit that serves to direct return fluid discharged from hydraulic actuators back to a tank.
In order to solve the above problems, an object of the invention is to provide a work machine of which a boom control circuit is adapted to function independently so that the flow rate required by the boom control circuit can be easily ensured. Another object of the invention is to provide a work machine wherein energy of return fluid discharged from hydraulic actuators can be effectively recovered even in an open circuit.